Lubricating greases are generally defined by two properties. One is the consistency of the grease. Because a lubricating grease is a non-Newtonian semi-solid material, viscosity cannot be measured in the same way that a measurement would be made for a liquid lubricant. Rather, the “consistency” of a grease refers to how stiff the grease is under prescribed test conditions. Grease consistency is measured by a cone penetration test. Such tests are defined by various standards such as ISO 2137, ASTM D217, or ASTM D1403. The results of this test allow a consistency class e.g. #2 to be assigned to the grease according to a classification system established by the NLGI (formerly known as the National Lubricating Grease Institute). Softer greases will generally have a higher penetration number according to cone penetration tests. Comparisons of grease properties are generally done for greases in the same consistency class.
The other significant property used to define a lubricating grease is the dropping point. The dropping point is the temperature at which grease becomes soft enough to allow oil and material to separate from the matrix of the grease and fall from the orifice of the testing apparatus. The dropping point of a grease can be measured by various tests, such as ISO 2176 (ASTM D566), ASTM D2265, or IP 396 Automatic Dropping point test. The dropping point may be indicative of the upper operating temperature of the grease.
Simple soaps are known to be used to thicken lubricating oils in order to make grease compositions. Simple soaps are usually defined as the reaction product of a single fatty acid with an alkali source. The fatty acids can be derived from natural oils from plant and animal sources. A common fatty acid derived from plant sources is oleic, while one from animal sources is stearic. Both of these C18 acids have a hydrocarbyl tails attached to a single carboxylic acid head group. Another commonly used fatty acid is 12-hydroxystearic acid. This fatty acid is derived from hydrogenating castor oil. Simple soaps generally have dropping points which are similar to the melting temperature of the simple soap.
The dropping point of simple soaps can be increased in a process known as “complexing” which involves reacting the simple soap thickener with a complexing agent, such as dicarboxylic acids with 6 to 12 carbon atoms, for instance sebacic (C10) acid or azelaic (C9) acid. Complexing with the diacids results in increased cost of the overall grease product and can negatively impact the flow properties of the grease especially, at low temperature.
Boron containing compounds have been used as dropping point enhancers for greases made with simple soap thickeners to replace the complexing diacids. Boric acid has been found to be difficult to incorporate into the grease and the resulting greases are not as thermally stable as those made with the dicarboxylic acids. Some low molecular weight (C4 to C8) borate esters have been found to be able to be incorporated into greases, but have other issues. Borate esters are generally hydrolytically unstable and readily react with moisture in the air, which liberates alcohols from the borates to also produce boric acid. The use of these borate esters causes strong alcohol odors in the finished grease.
Therefore, there is a need to provide a high dropping point, metal simple-soap thickened grease composition comparable to, or superior to, what is currently achievable via standard grease “complexing” technology, but which avoids the notable drawbacks of difficult incorporation, strong alcohol odors, and poor grease flow properties at low temperatures.